The effects of pH on recombinant human erythropoietin (rHuEPO) production by Pichia pastoris was investigated at pH = 4.0, 4.5, 5.0, 5.5 and 6.0, where sorbitol and methanol were supplied to the bioreactor in batch and fed-batch mode, respectively, in the recombinant protein production phase (PP). For the first three phases, glycerol batch (GB), glycerol fed-batch (GFB) and methanol transition (MT), the growth profiles were almost the same for pH >= 5.0; however, at pH < 5.0 the growth rate was lower. The highest cell concentration was obtained as 81.4 g L(-1) where GB and GFB were operated at pH = 5.0 and MT and PP were operated at pH = 4.5. Sorbitol added at the beginning of PP, was totally consumed within 15 h, at pH <5.0: however, at pH > 5.0 sorbitol consumption rate decreased. The highest rHuEPO concentration was achieved at pH = 4.5 as 0.16 g L(-1), where the overall cell and rHuEPO yield on substrate, were 0.5 g g(-1) and 1.5 mg g(-1). pH did not have a significant effect on extracellular protease activity. The highest specific AOX activity (110 U g(-1) CDW) was attained at pH = 4.5 parallel to the highest rHuEPO production. At pH = 4.5, higher oxygen demand of the cells resulted in higher oxygen K(L)a, 0.11 s(-1), values. (C) 2011 Elsevier B.V. All rights reserved.